Semiconductor devices are used in a large number of electronic devices, such as computers, cell phones, and others. Semiconductor devices comprise integrated circuits that are formed on semiconductor wafers by depositing many types of thin films of material over the semiconductor wafers, and patterning the thin films of material to form the integrated circuits. Integrated circuits include field-effect transistors (FETs) such as metal oxide semiconductor (MOS) transistors.
One of the goals of the semiconductor industry is to continue shrinking the size and increasing the speed of individual FETs. To achieve these goals, fin FETs (FinFETs) or multiple gate transistors are used in sub 32 nm transistor nodes. FinFETs not only improve areal density, but also improve gate control of the channel.
Multiple threshold voltages (Vth) are feasible for FinFET technology at 22 nanometer (nm) node geometries using a conventional channel doping method (e.g., through ion implantation and annealing).
One of the challenges faced when using conventional ion implantation prior to fin formation is the dopant loss in the subsequent process steps after fin has been formed due, at least in part, to the fins being relatively thin (e.g., 10-15 nm widths and 30-50 nm heights) with large surface to volume ratios. The dopant loss may cause random dopant fluctuations (RDF), which have a direct impact on threshold voltage controllability. Random dopant fluctuations may also be the result of a fin width/height variation, which will change the ratio of fin surface to fin volume.
For bulk silicon (Si) fins, another way of controlling multiple threshold voltages in FinFETs is to perform ion implantation after the fin has been formed. However, one of the challenges for this integration scheme is that after ion implantation an anneal step is needed to re-crystallize the fins. With free standing fins, which are amorphous after regular ion implantation at room temperature, the seed for re-crystallization of the fins is the remaining fin body inside of the oxide fillings. The re-crystallization process can introduce high levels of crystal lattice defects, which inevitably cause mobility degradation in fins.
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